1. Field of the Invention
The present invention relates to an exerciser for an emergency shutoff valve used with steam turbines.
2. Description of the Related Art
An emergency shutoff valve, or trip valve, is used on steam turbines to quickly shutoff the flow of steam, thereby shutting down the turbine. Different turbine conditions may trip the shutoff valve such as an over speed condition of the turbine or, in some cases, the existence of a low oil pressure condition.
Since an emergency shutoff condition occurs only rarely, the shutoff valve for most of its life is in the open position, whereby steam can freely travel between the inlet through an aperture and to the outlet. However, as a result of the valve remaining in the same position for a long period of time, deposits build up on the bushings surrounding the valve stem. It is imperative during a trip condition that the valve stem is free to move so that the valve closes without delay. The only certain way to check the integrity of the valve is to occasionally move the valve stem back and forth, otherwise known as exercising the valve, within the valve body so that deposits around the bushings are broken up and do not slow or bind the valve from closing. It is only necessary to reciprocate the valve stem within a small range, for example 0.25 inches, to exercise the valve.
A valve exerciser consists of a number of components that are assembled on the existing emergency shutoff valve and provide a means for an operator to move the valve stem with the valve plug in a controlled manner.
In the past, in order to exercise a valve, it was necessary for the operator to shutoff the flow of steam to the turbine and, at that time, reciprocate the valve stem within the valve body to break any deposits that may have built up. Taking a steam turbine off-line is a very costly and time consuming event and, for that reason, is done only when absolutely necessary.
It is an object of the subject invention to provide a device and a method of exercising the emergency shutoff valve while maintaining the shutoff valve in an operational condition such that the associated steam turbine may continue to operate while the valve is being exercised.